A carlactonoic acid methyltransferase that contributes to the inhibition of shoot branching in Arabidopsis

Abstract

Strigolactones (SLs) are plant hormones that regulate shoot branching and diverse developmental processes. They are biosynthesized from carotenoid molecules via a key biosynthetic precursor called carlactone (CL) and its carboxylated analog, carlactonoic acid (CLA). We have previously identified the methyl esterified derivative of CLA, methyl carlactonoate (MeCLA), as an endogenous SL-like molecule in Arabidopsis. Neither CL nor CLA could interact with the receptor protein, Arabidopsis DWARF14 (AtD14), in vitro, while MeCLA could, suggesting that the methylation step of CLA is critical to convert a biologically inactive precursor to a bioactive compound in the shoot branching inhibition pathway. Here, we show that a member of the SABATH protein family (At4g36470) efficiently catalyzes methyl esterification of CLA using S-adenosyl-L-methionine (SAM) as a methyl donor. We named this enzyme CLAMT for CLA methyltransferase. The Arabidopsis loss-of-function clamt mutant accumulated CLA and had substantially reduced MeCLA content compared with wild type (WT), showing that CLAMT is the main enzyme that catalyzes CLA methylation in Arabidopsis. The clamt mutant displayed an increased branching phenotype, yet the branch number was less than that of severe SL biosynthetic mutants. Exogenously applied MeCLA, but not CLA, restored the branching phenotype of the clamt mutant. In addition, grafting experiments using the clamt and other SL biosynthetic mutants suggest that CL and CLA are transmissible from root to shoot. Taken together, our results demonstrate a significant role of CLAMT in the shoot branching inhibition pathway in Arabidopsis.